1. Field of the Invention
This invention relates in general to sensors for magnetic storage devices, and more particularly to a method and apparatus for providing magnetostriction control in a free layer of a magnetic memory device.
2. Description of Related Art
Magnetic recording is a key segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was used for data storage. Areal density continues to grow due to improvements in magnet recording heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of the magnetic recording heads to both write and subsequently read magnetically recorded data from the medium at data densities well into the gigabits per square inch (Gbits/in2) range gives hard disk drives the power to remain the dominant storage device for many years to come.
Important components of computing platforms are mass storage devices including magnetic disk and magnetic tape drives, where magnetic tape drives are popular, for example, in data backup applications. Write and read heads are employed for writing magnetic data to and reading magnetic data from the recording medium. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor changes resistance in the presence of a magnetic field. Recorded data can be read from a recorded magnetic medium, such as a magnetic disk, because the magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the read element, which causes a corresponding change in the sensor resistance.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as MR read heads for reading data in magnetic recording disk and tape drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the magnetic disk in a magnetic disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element. This change in resistance may be used to detect magnetic transitions recorded on the recording media.
In the past several years, prospects of increased storage capacity have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. In a spin valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium, or signal field, causes a change in the direction of magnetization of the free layer, which in turn causes a change in the resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
Magnetic sensors utilizing the GMR effect are found in mass storage devices such as, for example, magnetic disk and tape drives and are frequently referred to as spin-valve sensors. In an AFM pinned spin valve, the pinned layer is magnetically pinned or oriented by an adjacent pinning layer. In a self-pinned spin valve, the magnetic moment of the pinned layer is pinned in the fabrication process, i.e., the magnetic moment is set by the specific thickness and composition of the film.
Recently, magnetic tunnel junction sensor devices have been proposed for a variety of applications, including read heads for magnetic disks as well as magnetoresistive random access memory. A magnetic tunnel junction (MTJ) is a type or magnetoresistive device made of at least two magnetic film layers separated by an insulating barrier. The insulating barrier is thin enough to allow electrons to quantum mechanically tunnel through the barrier. Resistance of an MTJ is directly related to the tunneling probability that depends on the relative orientation of the magnetization vectors of the magnetic layers. Because the orientation of the magnetization vector depends on the applied field, the resistance of a MTJ device varies in the presence of a magnetic field.
Spin valve sensors and MTJ devices include at least three layers of thin material that combine into a single structure. A free layer acts as the sensing layer. The free layer is passed over the surface of the data bits to be read. It is free to rotate in response to the magnetic patterns on the disk. A separation layer is provide adjacent the free layer. In a GMR sensor, the separation layer is a conductor, such as copper. In MTJ devices, the separation layer is an insulation layer, such as Al2O3. The pinned layer is a layer of material that is held in a fixed magnetic orientation as described above.
Free layer magnetostriction is one of the key parameters that need to be controlled for good sensor performance. A free layer is often formed using a bilayer structure, such as CoFe and NiFe. Currently the magnetostriction control for bilayer structure for the free layer is accomplished by changing the composition of a layer, e.g., the NiFe or CoFe layer. However, changing the composition of a layer is very time consuming and costly, but is often required as sensor designs change.
It can be seen that there is a need for a method and apparatus for providing magnetostriction control in a free layer of a magnetic memory device.